Many substrate treatment processes involve exposing a substrate in a vacuum chamber to an ion beam, thereby causing the substrate to absorb heat. Because the substrate can only absorb a certain amount of heat or reach a certain temperature before being damaged, a common problem encountered is how to cool the wafer during treatment. Typical treatment processes require that substrate temperatures be maintained below 100.degree. C. to be compatible with photoresist.
For treatment processes using pressures approaching atmospheric pressure, the density of gas atoms in the vicinity of the substrate is sufficient to absorb the heat from the substrate. As the pressure is reduced less heat can be removed via the gas in the treatment chamber. With Argon gas and a pressure of approximately 10.sup.-4 Torr, the mean free path of the Argon molecules is approximately the physical dimension of a typical process chamber (i.e. 150 cm). Under such conditions, the wafer can not be exposed to the ion beam for a significant period of time without being damaged.
With the pressure too low to provide effective substrate cooling in the treatment chamber, other heat transfer techniques are needed. At temperature approximating 100.degree. C., the temperature is too low for effective radiation cooling. With wafer surfaces typically allowing no more than 15% of the surface area to make actual contact with a cooled support surface, solid-to-solid conduction also may be ineffective.
For substrate treatment processes using an ion beam, substrate cooling has been a significant problem. An early approach was to clamp the wafers to improve the percentage of contact between the back surface of the substrate and a substrate holder or cooled platen. An improvement on this was to place compliant thermally conductive materials between the substrate and the platen. Another method was to employ beam sharing techniques where substrates are moved in and out of the beam to reduce heat build-up. However, demands for increased processing throughput resulted in treatment processes requiring increased beam current and power densities. Such increased power densities prompted a need for better cooling technology. Additional demands also occurred for substrates having increased diameter and for equipment having fewer failures attributable to particle contamination.
In the ion beam etching field, cooling techniques progressed from grease to thermally conductive rubber pads and electrostatic hold down plates. These techniques have been effective for power densities approximating 500 mW/cm.sup.2, but are operator intensive requiring careful attention to detail to achieve reliable results.
An improvement has been to use gas contained between the substrate and the cooled substrate support to enhance the solid-to-solid conduction with thermal conduction by molecular gas heat transfer. In thermal conduction, gas molecules in contact with the back surface of the substrate absorb heat then travel without collision to the cooled support and transfer a portion of the absorbed heat to the cooled support. A problem with such a contained gas is that the gas heats up over time and becomes ineffective for treatment processes of long duration such as ion beam etching processes.
To enable gas cooling to be effective for prolonged substrate treatment processes, it is an object of this invention to increase the gas pressure to increase the number of gas molecules and to decrease the mean free path of the molecules to a value less than the distance between the substrate and the support such that cooling is achieved by free convection.
It is another object of this invention to flow the gas into and through the space between the substrate and the cooled support to achieve heat transfer by forced convection.
It is another object of this invention to achieve reliable loading and unloading of a substrate from a reliable and modular substrate fixture while under substantially vacuum pressure conditions.
It is another object of this invention to provide a substrate cooling fixture which tilts and rotates during ion beam etching processes, acts as a heat sink for the substrate and is compatible with automatic substrate loading and unloading processes.
It is another object of this invention to achieve high throughput ion beam processing by providing a substrate cooling fixture and a load-locked vacuum chamber within a system for transporting a wafer between the fixture and the chamber.
It is another object of this invention to achieve low particular generation to improve the yield of fabricated substrates.
It is another object of this invention to provide a substrate loading station that is compatible with cassette to cassette operations.
It is another object of this invention to provide a system combatible with automatic material transport systems.